On-demand page printers, wherein images are created in response to digital image data submitted to the printing apparatus, are familiar in many offices. Such printers create images on sheets typically using electrostatographic or ink-jet printing techniques. In work-group situations, wherein different users at various personal computers and other terminals submit jobs to a single central printing apparatus, various sets of digital image data, corresponding to jobs desired to be printed by different users, are typically kept in an electronic queue, and a control system typically located at the printer sorts through the image data and causes the printer to output the desired prints in an orderly manner.
Particularly with sophisticated printing apparatus, it may often be desired to print "duplex" prints, that is prints having images on both sides of the sheet. However, just about every currently commercially available printing apparatus is capable of producing an image only on one side of a sheet at a time. In order to obtain duplex prints, it is almost always necessary to provide an "inverter" within the printing apparatus. The purpose of an inverter is to handle a sheet after one side thereof has received an image, and in effect turn the sheet over to make the remaining blank side available to the same printing apparatus which created the first image. In effect, each duplex print is re-fed past the image-making portion of the printing apparatus so that the individual sheet becomes available to the image-making apparatus twice, once for each side.
A long-standing concern of designers of printing apparatus is how to optimize the use of a printing apparatus for situations wherein some desired prints are simplex and others are duplex. The fact that each duplex print has to be printed essentially twice causes a significant systemic problem with maintaining optimal or near-optimal operation of the entire printing apparatus. One simple solution, for example, would be to run every sheet along the duplex path, regardless of whether it is a simplex or duplex print, and in the case of each simplex print simply print nothing on the back side. While this solution is easy to implement, it provides the disadvantages of unnecessarily decreasing the output speed of the whole system. Another solution is to maintain duplex prints which are awaiting printing on the back sides thereof in a special buffer tray, until the system becomes available for printing the back sides of each sheet in sequence. The key disadvantage of this system is that a significant probability of error exists (a sheet may have the incorrect back side image placed thereon), and also the relatively intense handling of each print sheet in and out of the buffer tray substantially increases a likelihood of mechanical misfeed. Both such problems tend to result from the fact that sheets typically cannot be fed out of the buffer tray reliably. Even with a buffer tray, a fairly sophisticated scheduling system is required.
In electrostatographic printing apparatus, wherein images are first created on a photoreceptor in the form of a rotating drum or belt and then transferred to sheets, a key concern is the presence of blank pitches (image-sized spaces) along the drum or belt where, for various reasons relating to duplexing, no image is created. The problem with blank pitches is that each blank pitch represents lost productivity. In some duplexing schemes, the number of blank pitches along the belt may be comparable to the number of pitches actually having images on them. In such a situation, not only is the apparatus effectively running at half-speed, but various mechanical parts associated with the drum or belt will be experiencing wear to no productive purpose. Thus, as a general rule, the overall productivity of such printing apparatus is closely related to the number of blank pitches which result in the printing process.
In the prior art, certain patents, which are assigned to the assignee hereof and which are above incorporated by reference, have generally dealt with the problem of scheduling sheets in an imaging system having an endless duplex paper path loop. In general, these various patents disclose different techniques for overcoming the simplex-duplex problem in electrostatographic printing apparatus. Such techniques include simply finding blank pitches and using them as needed to print simplex prints or cycling down the printer between jobs where a finishing activity, such as stapling, requires an extraordinarily long time. Another technique involves causing sheets to move through the system at variable speeds. In contrast to these various systems, the present invention posits a system in which an optimized schedule for imaging sheets, given the knowledge of the desired output at a given time, is dynamically created in advance in the course of the printing process.
U.S. Pat. No. 5,095,371 discloses a general overview of a simplex-duplex printing system wherein various modular units, such as finishers, auxiliary paper feeders, and mail box units, each include a CPU which is connected to a central control system through an optical fiber cable.